As the West's main negotiator with Russia, the German leader has tried to redefine power in Europe. Yet her patience, restraint, and step-by-step diplomacy are being tested by Putin. Germany must be able to show how the Continent can live in peace.

To Angela Merkel, the leader of Germany, the main cause of Europe’s many wars has been simply this: rivalry among nations. To maintain peace, she said last March, a nation must confine its interests to a larger good or it “will harm itself sooner or later.”

For almost a year, as Russia’s rivalry with Ukraine has escalated, the German chancellor has applied that logic in more than two dozen phone calls with Russian President Vladimir Putin. Throughout the crisis, the United States and others have let her take the lead with Moscow. She has been admired for her patience, step-by-step diplomacy, and insistence on avoiding a military solution.

In applying lessons from its own 20th-century past, Germany has now emerged as an indispensable leader. And coming on the heels of her critical role during the Continent’s euro crisis in 2010-11, Ms. Merkel has set down a leadership style worthy of a Nobel Peace Prize.

Yet her methods are still being tested by Russia. First, Mr. Putin took Crimea by force in March. Then the Ukrainian separatists he supports shot down Malaysia Airlines Flight 17 in July. And finally some 1,000 Russian forces entered eastern Ukraine in August. So far, more than 2,600 people have been killed in Ukraine.

Her tactics of forbearance and of not shaming Putin with defeat are now in question. Germany’s allies have pressed her to accept the stationing of NATO forces on the border with Russia. And Europe plans even tougher economic sanctions on Moscow.

Merkel has also been forced to stand up to German business leaders who have strong ties with Russia. “Being able to change borders in Europe without consequences and attacking other countries with troops is, in my view, a far greater danger than having to accept certain disadvantages for the [German] economy,” she told the German parliament Sept. 1.

In addition, she had to rein in Ukraine last month as its forces began to dominate its battles with pro-Moscow rebels. A victory by Kiev might have led to a full Russian invasion. Again, a key player was asked to tamp down its rivalry. (Germany also offered to treat 20 Ukrainian soldiers wounded in the fighting.)

All of her steps are helping to redefine power and strength for a continent that has long viewed those words in terms of brute force. Russia still holds that view, whereas Merkel sees power in the ability of nations to rise above their rivalry and embrace broader principles, such as military restraint, international law, and persistent dialogue.

She says Germany has a “historic responsibility” to guard Europe’s stability. Will her approach eventually win over Putin? Events in Ukraine will probably play out for some weeks. But the world has a stake in Merkel’s style of leadership, even as she finds her own way.